The described invention relates in general to a system and apparatus for mixing viscous substances such as dough, and more specifically to a cooling jacket incorporated into the mixing bowl component of industrial mixers for controlling the temperature of the substance being mixed during the mixing process.
Friction and viscous shear encountered during mixing typically causes a temperature rise in a substance being mixed. This temperature rise becomes more severe as mixing speed increases and can adversely affect a mixing process by making the substance sticky and difficult to process. Accordingly, mixers, particularly dough mixers, are most effective when equipped with some type of temperature control means, whereby the temperature of the substance to be mixed may be stabilized at a predetermined level or maintained below a predetermined threshold. For example, bread dough should be mixed at a temperature of about 78-80° F. A known means for controlling the temperature of a substance being mixed is through the use of a refrigeration jacket attached to the mixing bowl component of a mixer. Bowl refrigeration jackets, also referred to as “cooling jackets” usually include multiple coolant channels that are arranged perpendicular to the ends of a mixing bowl, and which are arrayed around the profile of the mixing bowl. Additional coolant channels may be optionally included on the ends of the mixing bowl.
Large commercial scale dough mixers may be manufactured both with and without mixing bowl cooling jackets based primarily on the type and quantity of dough to be mixed. Dough mixers manufactured with bowl cooling jackets are categorized as having either “indirect” or “direct: cooling. An indirect refrigeration system utilizes cold water, glycol, or brine as a cooling fluid. This cooling fluid is first chilled by a compressed refrigerant system separate from the mixer, and is then pumped to the mixer. The cooling fluid then circulates through the mixing bowl's cooling jacket, which is typically comprised of a series of parallel channels fastened directly to the exterior of the mixing bowl. Heat generated during the mixing process is transferred from the dough, through the material of the mixing bowl, and then into the cooling fluid. After the mixing process is complete, the cooling fluid is then piped back to a storage tank for reuse. This principle may be applied to a “direct” refrigeration system, as well. A direct expansion refrigeration system introduces refrigerant directly into the refrigeration jacket of a mixer to remove excess heat from the dough being mixed. This type of cooling system typically includes a compressor, a condenser, an evaporator, and a receiver. The bowl refrigeration jacket serves as the evaporator in this configuration and the types of refrigerants used in this configuration typically include R134a and MP-39.
Large commercial scale dough mixers may be manufactured both with and without mixing bowl cooling jackets based primarily on the type and quantity of dough to be mixed. The performance, i.e., cooling capacity, of a refrigeration system used with a commercial scale mixer is the ability of the mixing bowl cooling jacket to remove heat from within the mixing bowl during a batch cycle. As previously indicated, major sources for heat generated during the mixing process are dough ingredient temperatures, ambient temperatures around the mixer, and heat generated from friction and shearing forces within the mixing bowl as the dough is processed. In some circumstances, these variables make it difficult or impossible for a mixing bowl cooling jacket to provide adequate cooling. The heat transference in this type of system includes both conduction and convection. Convection is defined by the equation q=hAΔT, where h is the fluid convection coefficient (BTU/sec·in2·° F.), and A is the surface area (in2) in contact with the cooling fluid. The convection coefficient is determined by factors that include the fluid's composition, temperature, velocity, and turbulence. In dough cooling applications, the convection coefficient is most easily increased, and the temperature of the dough thereby decreased, through an increase in the cooling fluid velocity or cooling fluid turbulence. Thus, because increasing the convection coefficient is an effective means for lowering the temperature in certain mixing systems, there is a need for a mixing bowl refrigeration system that includes means for, at a minimum, increasing the convection coefficient during the mixing process.